The invention relates to a method of producing an elastomeric alloy that is similar to thermoplastic elastomers using reclaimed rubber or waste rubber, in which the reclaimed rubber and waste rubber which has been converted into powdered rubber is processed with a thermoplastic and at least one stabilizer in various mass ratios by melt-mixing in a mixer to give compounds.
As a contribution to the solution of the problem of mechanical recycling of reclaimed rubber and waste rubber, a great deal of effort has been directed at the development of methods and formulations for the use of powdered rubber and granulated rubber from reclaimed or waste rubber. Such powdered rubber or granulated rubber is being offered to an increasing extent on the European market. Research work is concerned with the production, processing and testing of compounds that are similar to thermoplastic elastomers (TPEs) and are derived from powdered rubber and plastics.
The production of such elastomeric alloys by means of melt mixing with dynamic stabilization of the mixture in a mechanical mixer is known. The mechanophysical property profile of these compounds is determined by the plastic deformation behavior of the usually partially crystalline polymer matrix and the entropy-elastic, i.e. rubber-like, deformation behavior of the elastomeric domains dispersed therein.
Thus, for example, DE 295 15 721 U1 discloses the mixing of powdered rubber into polypropylene. The publication describes a thermoplastic, preferably polypropylene, styrene-butadiene-styrene or styrene-ethylene-butylene-styrene, modified with ground rubber as a substitute for or blending component for thermoplastic elastomers or impact-modified thermoplastics, which is characterized in that from 10 to 80 percent by weight of finely ground rubber having a particle size of  less than 600 xcexcm is physically incorporated in the matrix of the thermoplastic. Additional chemical bonding is achieved by means of peroxidic crosslinking or acid functionalization.
However, both the process technology employed there and the formulation are not sufficient for producing a level of mechanophysical materials properties which is similar or comparable to that typical for known elastomeric alloys.
While DE 295 15 721 U1 uses antioxidants for protection against light and heat, antioxidants are, in further known processes, of great importance for significantly increasing strength and hardness values by means of a donor-acceptor reaction.
Thus, DE 196 07 281 A1 discloses methods and formulations by means of which it is possible to produce elastomer compounds which have a level of materials properties, in particular mechanophysical properties, which is significantly improved compared to the known prior art. According to this publication, the method is based on converting the mixture of at least two intrinsically incompatible mix constituents, namely powdered rubber and thermoplastic, into a compound having good impact toughness, by creation of appropriate materials, technological and constructional conditions during the melt-mixing process. For this purpose, untreated and/or activated powdered reclaimed or waste rubber and a thermoplastic component are introduced into a mechanical mixer, the thermoplastic component is plasticized and at the same time the powdered rubber is dispersed or mixed in. Addition of one or more crosslinkers in various mass ratios and the action of high shear forces result in dynamic stabilization of the elastomer and/or thermoplastic components and conversion into compounds having properties similar to those of thermoplastic elastomers.
A disadvantage of this solution is that homogeneous distribution of the crosslinker is not ensured and, as a result, stable reproducibility of equal quality compounds over a longer period of time is not guaranteed.
It has also been found that not every thermoplastic material is equally suitable for producing high-quality and multiply recyclable compounds, since the achievable degree of crosslinking of the thermodynamically incompatible components is very different.
It is therefore an object of the invention to improve known methods of mechanical recycling of reclaimed rubber and waste rubber by dynamic stabilization of powdered rubber, thermoplastic and crosslinkers in the manner described at the outset so that a mixture of two intrinsically incompatible phases, namely powdered rubber and thermoplastic, is converted during a relatively short mixing time in a melt-mixing process into an alloy which has properties similar to those of thermoplastic elastomers and is of constant, reproducible quality and is multiply recyclable by appropriate materials selection for the thermoplastic component and further additives and by improved technology of the process. In particular, the compounds should have rubber-like materials properties and constant good quality.
According to the invention, this object is achieved in that at least one polypropylene copolymer or a mixture thereof with at least one polypropylene grade is firstly melted in a mixer and powdered rubber of which at least part has been preswelled in a free-radical donor is metered into the melt, in that the powdered rubber is dispersed in the polymer matrix by application of high shear deformations and with mixing-parameter-dependent addition of free-radical-forming agents for phase coupling between the powdered rubber and the polypropylene copolymer or mixture thereof and in that the dynamic stabilization process is carried out at a mix temperature which is above the melting point range of the polypropylene copolymer or mixture thereof but below the decomposition temperature range of the powdered rubber and at a mixing time which allows reaction of the free-radical former.
These elastomeric alloys which can be produced by the method of the invention combine the advantages of thermoplastic processability with rubber-like materials properties. These elastomeric alloys are very similar to thermoplastic elastomers. A polypropylene copolymer having a proportion of polyethylene or a mixture of partially crystalline polypropylenes (homopolymer and copolymer) functions as polymer phase. Since the method concentrates on the mechanical recycling of reclaimed rubber and waste rubber, the formulation development is based on a mixing ratio of powdered rubber to polymer of at least 50% of powdered rubber. The elastomeric alloys can be produced reproducibly in terms of their mechanophysical property profile and have a property structure which can be correlated amongst them.
The elastomeric alloys are recyclable, i.e. they can, in contrast to rubber, be processed a second or further time without appreciable loss of properties.
The novel aspect of the procedure is the introduction and utilization of the affects of dynamic stabilization of the polymer mixture. High-quality bonding of the two phases can be achieved only by a combination of mechanical energy input and chemical reaction after preswelling of the disperse phase. The polyethylene component migrates to the phase boundary and displays crosslinking and coalescence effects there. Thus, high proportions of, for example, powdered reclaimed tires having a coarse structure (particle size at least 0.5 mm) can be used and converted into compounds having very good mechanophysical properties. The preswelling of the powdered rubber serves to restrict the attack of the free-radical donor on the phase boundaries.
The mixture of powdered rubber, a specifically selected polymer and the stabilizer or the additives is brought, in a mixer suitable for applying high shear deformations, here a mechanical mixer, to an energy level at which the thermoplastic component is melted and the elastomer component is dispersed in it. The achievable property spectrum of these compounds can be varied within wide limits as a function of the formulation, i.e. the mass ratio of powdered rubber to polymer and further additives, and also the mixing technology and can be tailored to the respective use of the polymer alloy.
The compounds can be used as molding compositions, in particular for industrial moldings. Owing to their good mechanophysical properties, particularly in respect of their tensile strength and elongation at break and also their resistance to impact and shock even at low temperatures, they can be used for industrial molded articles subject to high stresses and can thus provide an inexpensive and environmentally friendly replacement for conventional thermoplastic elastomers comprising a pure polymer with a crosslinked or uncrosslinked pure rubber phase.
Typical examples of applications are semifinished parts or moldings which as sleeves and seals and also shock absorbers and linings in motor vehicle and rail vehicle construction. Containers, buckets or covers in road and pipeline construction, produced by injection molding, extrusion, rolling, calendering or pressing, can also be manufactured inexpensively from these compounds.
In an advantageous embodiment of the method of the invention, at least a proportion of at least one polypropylene homopolymer is added during melting of the polypropylene copolymer.
In this way, hardness and strength, in particular the abrasion resistance, of the stabilized system can be increased further.
Furthermore, the invention provides for use of a polypropylene blend comprising a mixture of copolymer and homopolymer and having a copolymer content of up to 95% of the total polymer matrix as thermoplastic.
This, too, can improve the mechanophysical properties, here impact strength when used at low temperature and elongation at break at room temperature. The compounds have significantly better flowability, which aids processing conditions, particularly in injection molding and extrusion.
If the free-radical donor used is a liquid peroxide having a high proportion of active oxygen and a weight ratio to the polymer component of  greater than 0.3%, this leads to improved crosslinking of the components of the mixture.
The high proportion of active oxygen contributes to a very active reaction. The effective surface area of the dispersed elastomeric phase is increased by swelling and free-radical formation is effected at the powdered rubber surface by means of partial breaking of crosslinks or cleavage of double bonds still present in the rubber. The proportion of polyethylene present in the copolymer is crosslinked by means of the peroxide. The degree of grafting is a function of the reactivity of the peroxide.
In terms of the achievable mechanophysical properties, it is also advantageous for the matrix to be exclusively a polypropylene-co-polyethylene.
The surface tensions of the materials to be mixed should generally be very close to one another. Polyethylene has a significantly lower surface tension than polypropylene and is far closer to the surface tension of natural rubber as constituent of the powdered rubber. It is therefore better able to bond to the powdered rubber.